The proposed research involves structural studies with an enzyme from the reductive acetyl-CoA pathway for CO 2 fixation. Clostridium thermoaceticum, an anaerobic acetogene, is the model system for this unusual pathway. Determining this MeTr structure will also shed light on the structure and function of other closely related cobalamin-dependent methyltransferases: those from methanogenic bacteria and the methyl transfer in the first half reaction of B 12 -dependant methionine synthase enzymes.